Inheritance Flashcards

All 34 cards in this deck

What is a chromosome?

A long, coiled molecule of DNA found in the nucleus, carrying many genes.

What is an allele?

A different version of the same gene.
e.g. the flower colour gene has a purple allele and a white allele.

What is a dominant allele?

An allele that is expressed in the phenotype even when only one copy is present.

What does homozygous mean?

Having two identical alleles of a gene.
e.g. $DD$ or $dd$

What is a genotype?

The alleles an organism has for a characteristic.
e.g. $Pp$

What is a gamete?

A sex cell (sperm or egg) that carries one allele of each gene.

What are the steps to complete a Punnett square for a monohybrid cross?
e.g. $Dd \times Dd$

1. Write one parent's gametes along the top: $D$ and $d$
2. Write the other parent's gametes down the side: $D$ and $d$
3. Combine the letters in each box: $DD$, $Dd$, $Dd$, $dd$

Two purple-flowered plants produce some white-flowered offspring. What does this tell you about the parents' genotypes?

Both parents are heterozygous ($Pp$) — each carries an allele for white flowers, so some offspring are homozygous recessive ($pp$).

What is a carrier of a genetic disorder?

A heterozygous person who has one recessive allele for the disorder but does not have the disorder, and can pass the allele on.

What are the sex chromosomes of a human male and a human female?

Male: XY. Female: XX.

Which parent determines the sex of a child, and why?

The father — his sperm carry either an X or a Y chromosome, while all egg cells carry an X.

What is the probability that a child will be male?

0.5 / 50% / 1 in 2.

True or false? If a couple already have three daughters, their next child is more likely to be a boy.

False. Each fertilisation is independent, so the chance is still 50%.

What is polygenic inheritance?

When a characteristic is controlled by several genes working together, rather than by a single gene.

Are most phenotypic features controlled by one gene or by several genes?

Several genes — most features are polygenic.

True or false? Human height is controlled by a single gene.

False. Height is polygenic — it is controlled by many genes (and also affected by the environment).

What is variation?

Differences in the characteristics of individuals of the same species.

State two causes of genetic variation.

Mutation, and sexual reproduction giving different combinations of alleles.

What is environmental variation?

Differences in characteristics caused by an organism's environment (acquired characteristics).
e.g. a scar, or a plant growing poorly in shade

True or false? Eye colour is an example of environmental variation.

False. Eye colour is inherited, so it is genetic variation.

What did the Human Genome Project do?

It worked out the sequence of all 3 billion base pairs in human DNA, finishing in 2003.

What is personalised medicine?

Treatment or medicine chosen to suit an individual patient's own genes.

What is a mutation?

A random change in the base sequence of DNA. A mutation within a gene can produce a new allele.

What is the original source of the extensive genetic variation within a population of a species?

Mutations. There is usually extensive genetic variation within a population.

What effect do most mutations have on the phenotype?

No effect at all; some have a small effect.

True or false? A single mutation can sometimes significantly affect the phenotype.

True, but this is rare.

What is a recessive allele?

An allele that is only expressed in the phenotype when two copies are present.

What does heterozygous mean?

Having two different alleles of a gene.
e.g. $Dd$

What is a phenotype?

The observable (physical) characteristics of an organism.
e.g. the colour of a carrot

What is a zygote?

The single cell formed when two gametes fuse at fertilisation.

Is most variation in phenotype caused by genes, by the environment, or by both?

Both. Most phenotypic variation comes from genes and the environment acting together.
e.g. a person's body mass depends on their genes and their diet.

Give one medical benefit of the Human Genome Project.

Genes that raise the risk of a disease can be identified, so people can be tested and the disease prevented or treated early.
Accept: choosing the best drug for a patient's genes (personalised medicine).

Give one possible problem with people knowing what alleles are in their genome.

Someone who learns they carry a high-risk allele may suffer worry or stress.
Accept: discrimination, e.g. by employers or insurers.

What ratio of phenotypes is expected from crossing two heterozygous parents (e.g. Dd × Dd)?

3 : 1, dominant to recessive. So there is a 25% (1 in 4) chance of an offspring having the recessive phenotype.